1. Field of the Invention
The present invention relates to an image processing technique applied for images projected by a projector.
2. Description of the Related Art
A projector modulates light emitted from a light source based on image data with its light modulation means, such as a liquid crystal panel, and displays a resulting image on a screen. ‘Elevated projection’ is often adopted to prevent any obstacle between the projector and the screen from interfering with the view of the projected image.
FIG. 11 shows projection of an image on a screen SC with a prior art projector 100. In the example of FIG. 11A, an elevation angle, that is, an angle defined by an optical axis of the projector 100 and a normal line of the screen SC, is equal to 0. A resulting projected image P is rectangle without any significant distortion. In the example of FIG. 11B, the elevation angle is θ (≠0). A resulting projected image P′ has trapezoidal distortion or keystone distortion.
The image processing technique to correct the trapezoidal distortion to a rectangle has been widely used.
Elevated projection may be performed in the horizontal direction. In the specification hereof, the ‘elevated projection’ includes inclination of the optical axis of the projector in the vertical direction relative to the normal line of the screen SC and inclination in the horizontal direction. FIG. 12 shows elevated projection at an elevation angle θ′ in the horizontal direction with the prior art projector 100. In this case, the image processing is carried out to correct the shape of a resulting projected image P″ to a rectangle.
Adjustment of the focal distance of the projection lens in the projector is required for clear projection of the image on the screen SC. There is, however, strictly only one focalized site on the screen SC, and the other sites are in slight out-of-focus state. The projected image at such sites is out of focus and rather blurred. The deterioration of the image quality due to the out-of-focus state is especially distinctive under the condition of a large elevation angle and in front projectors where the projection distance and the elevation angle are arbitrarily settable.
The deterioration of the image quality is not restricted to the elevated projection, but is commonly observed in any projection of the image. Any improvement of the optical system can not sufficiently solve this problem.